The invention relates to a method and apparatus for transmitting data over a line of an alternating current distribution network, in which a current signal typical of the data to be transmitted is generated and input into the network.
A method of this type is intended, for example, to report back the counter reading from counters distributed over the network, or information about executed commands from ripple control receivers, to a central unit, such as used in the textile industry. This remote indication requires a large number of transmitters which as far as possible should be inexpensive, and have a relatively low power rating, whereas the central receiver may tend to be expensive. The signal evaluated by the receiver is either the current signal itself or a signal derived from this, such as, for example, the resulting voltage drop at the leakage inductance of the transformer feeding the network.
In conventional methods of this type, two of which are described in, for example, German Patent Specification No. 2,820,213, and Swiss Patent Specification No. 540,607, the transmitter contains a network which is connected to the power supply terminals either directly or via a controllable switch. This network is a frequency-selective, tuned resonant circuit, one of the factors determining the resonant frequency being the desired transmission frequency. Moreover, in the method according to Swiss Patent Specification No. 540,607, the transmission signal generated in a power stage is coupled into the network via a transformer.
Apart from the fact that the power required for the current signal is first taken from the network and then returned to the network, the frequency-selective network constitutes a severe restriction in that it makes it difficult to modify the transmission frequency during operation. The active part of the transmitter is formed by a suitable amplifier stage, which has to be protected from high voltages by appropriate filters.